Filter Expression Autocomplete

[rehandalal]

Copied from: mozilla/normandy#979

Description
Provide users editing the extra filter expression field with autocompletion for objects available in the JEXL context (normandy, transforms, and operators)

Goal

• Eliminate the need to constantly refer to the filter expression docs
• Prevent typos, forgotten arguments, etc, ultimately saving time for the end user

Level of Effort
Medium-. The front end library we use has components that come real close to what we want, but the last 10% to nail it is 90% of the work.